Developments of the fourth generation mobile communication scheme that is a mobile communication scheme of a next generation of IMT-2000 (International Mobile Telecommunication 2000) are being progressed. In the fourth generation mobile communication schemer it is desired to support from multi-cell environments such as cellular systems to isolated-cell environments such as hot spots or indoor areas, and further desired to increase frequency use efficiency in both cell environments.
As a candidate of a radio access scheme applied for a link (to be referred to as “uplink”) directed from a mobile station to a base station in the fourth generation mobile communication scheme, DS-CDMA (Direct Sequence-Code Division Multiple Access) is promising. In the direct sequence-code division multiple access, a transmission signal is multiplied by spread code so that it is transmitted while being spread into a wideband signal (refer to non-patent document 1, for example).
However, since the DS-CDMA is a radio access scheme suitable for a multi-cell environment, the following problems are worrisome. That is, in an isolated-cell environment such as a hot spot area and an indoor area where influence of other-cell interference is normally small, there are few advantages in decreasing the other-cell interference by using spreading. Therefore, in DS-CDMA, in order to realize frequency use efficiency similar to that of TDMA, it is necessary to accommodate many signals.
For example, when each mobile station transmits a transmission signal by multiplying the transmission signal by spread code of a spreading factor SF, information transmission speed becomes 1/SF. Thus, for realizing the frequency use efficiency the same as that of TDMA, it is necessary to accommodate SF signals of the mobile station in DS-CDMA. However, in an actual radio propagation environment in an uplink, influence of multiple access interference (MAI) in which signals from each mobile station interfere with each other become dominant due to difference of propagation conditions from each mobile station to the base station (variation of propagation delay time and propagation path, for example). As a result, the frequency use efficiency normalized by the spreading factor is decreased to about 20%-30%.
On the other hand, as a radio access scheme that can reduce the above-mentioned MAI, IFDMA (Interleaved Frequency Division Multiple Access) is being studied (refer to the non-patent document 2, for example). In the IFDMA, information symbols are rearranged by applying symbol repetition to the information symbols such that a symbol pattern is generated, and they are transmitted by multiplying the transmission signal by a phase specific to the mobile station.
For example, as shown in FIG. 1, a data modulated symbol sequence is converted to blocks each for every Q symbols, and compression and SRF times repetition is performed. Accordingly, comb-shaped frequency spectrum can be generated. In addition, in IFDMA, by generating the symbol pattern and by performing multiplication of the phase specific to the mobile station, signals from each mobile station can be placed such that they do not overlap with each other on a frequency axis. Thus, MAI can be reduced.
VSCRF (Variable Spreading and Chip Repetition Factors)-CDMA is proposed as an radio access scheme based on symbol repetition of IFDMA (refer to non-patent document 3, for example). In the VSCRF-CDMA, chip repetition is applied to chips obtained by spreading the data modulated symbol sequence, and spreading factor for so-called time spreading and a chip repetition factor are adaptively updated according to cell configuration, a number of simultaneously accessing users, and propagation channel conditions.
The spreading and the chip repetition in the VSCRF-CDMA are described with reference to FIG. 2. A data modulated symbol sequence as a modulated transmission signal is multiplied by spread code of a spreading factor SF so that a chip sequence after spreading is generated. Next, the chip sequence after spreading is converted into blocks each for every Q chips for performing chip repetition, and compression and CRF (Chip Repetition Factors) times repetition are performed.
The chip sequence after the chip repetition shows a frequency spectrum on the frequency axis. Since the chip sequence is a signal having a chip pattern, the frequency spectrum becomes a comb-shaped spectrum.
In addition, by providing phase rotation specific to each user to the sequence after chip repetition, it becomes possible to assign a different comb-shaped frequency spectrum to each user so that signals of each user can be made orthogonal in the frequency domain.
For example, the sequence is multiplied by a phase vector s(k) specific to the user in order to assign comb-shaped frequency spectrums that are orthogonal among simultaneously accessing users. As shown in FIG. 3, a component of s(k) is represented as the following equation,st(k)=exp [−j·Φ(k)·t]wherein Φ(k) indicates a phase specific to a user and is represented by the following equation.Φ(k)=k×2π/(Q·CRF·Tc)In the equation, k indicates a user number, t=0, 1, 2, . . . , CRF×(Q−1).
As a result, since signals among CRF users at the maximum do not interfere with each other, that is, since there is no multiple interference, it becomes possible to receive signals of each user with high quality.
[Non-patent document 1] H. Atarashi, S. Abeta, and M. Sawahashi, “Broadband packet wireless access appropriate for high-speed and high-capacity throughput,” IEEE VTC2001-Spring, pp. 566-570. May 2001
[Non-patent document 2] M. Schnell, I. Broek, and U. Sorger, “A promising new wideband multiple-access scheme for future mobile communication systems,” European Trans, on Telecommun (ETT), vol. 10, no. 4, pp. 417-427, July/August 1999
[Non-patent document 3] Goto, Kawamura, Atarashi, Sawahashi, “Uplink Variable Spreading and Chip Repetition Factors (VSCRF)-CDMA broadband radio access”, IEICE Technical Report, RCS2003-67, July, 2003.